The present invention generally relates to insulated concrete wall panels, and more particularly to shear ties or connectors for such panels.
“Leadership in Energy and Environmental Design” (LEED) certified, environmentally-friendly construction has become standard practice in the United States. Building owners are willing to pay a premium overhead cost in order to save on life-cycle costs and receive additional financial benefits from the government by meeting LEED standards. To minimize operational costs and obtain LEED certification, building cladding systems must be thermally efficient. The precast concrete industry responded to consumer needs by developing the insulated precast wall panel, a building envelope comprised of a layer of insulating foam, typically expanded or extruded polystyrene, sandwiched between an external and internal layer (wythe) of concrete as shown in FIG. 1. If a solid concrete panel was fabricated with the same thickness as an insulated panel, the solid concrete wall would be over 50 times less thermally efficient than the insulated wall panel due to the high conductivity of concrete.
Intuitively by replacing concrete with foam reduces the strength of the wall panel. By removing the interior concrete, the mechanism to transfer the required interface shear is no longer present thus reducing the out-of-plane flexural strength of the panel. In order for insulated wall panels to have the same strength as a solid concrete wall panel, the interior and exterior concrete wythes must act in unison (compositely) to resist applied loads. The term “composite behavior” is a term of art referring to the ability of the insulated wall panel to behave under load as a single unitary structure. Historically, to achieve composite behavior strong metal devices (shear ties) physically connected the exterior and interior concrete wythes as shown in FIG. 1. The devices were able to transfer some horizontal shear to increase the strength of the panel; however, by physically connecting the two wythes with highly conductive metal devices, thermal bridging occurs creating “hot-spots” that had a detrimental effect on the thermal resistance of the entire panel. Additionally, localized hot-spots cause excessive condensation which can cause mold.
Today, several proprietary shear tie systems exist that utilize thermally resistive material to mitigate thermal bridging and condensation effects. However, tie systems available on the market today may lack the strength required to economically reach composite action, lack deformation capacity, allow concrete to pass along the tie creating a thermal bridge, or are subject to large installation tolerances creating a high variance in the actual panel strength.
Although insulated panels are designed utilizing shear ties to meet a prescribed demand and thermal resistance, they often suffer from quality control problems during fabrication. For shear ties to reach the capacity for which they are designed, they must be placed at specific locations and with defined embedment otherwise the capacity of the ties may be controlled by the strength of the concrete, due to pull-out or pry-out failure mechanisms, rather than the strength of the tie. Additionally, in poorly executed installations concrete may bleed past the shear tie creating a large thermal bridge reducing the overall panel thermal resistance. To ensure proper installation, extra effort must be taken by the fabricator during shear tie placement translating to a longer production time and additional expense.
Finally, shear ties currently available are designed to withstand conventional life cycle loads such as lifting during construction and wind loads during service; however, no shear ties exist which account for extreme events such as a blast load. Most government facilities mandate both energy and security requirements. An ideal solution to both requirements is a precast insulated wall panel with shear ties designed for high explosive detonations. In blast design, the primary mode of energy dissipation is panel deformation. By designing a shear tie with a large amount of ductility, the wall panel can displace more energy generated during a detonation before failing reducing the risk to inhabiting occupants.
An improved shear tie for a concrete wall panel is desired.